The normal sequence for recovering petroleum from subterranean petroleum deposits ordinarily comprises a first phase, referred to as primary recovery, in which petroleum is recovered by pumping or permitting the petroleum to flow to the surface of the earth through wells penetrating the formation and in fluid communication therewith, utilizing natural energy within the petroleum reservoir. Once this phase is completed, a secondary production phase is frequently applied, in which water is injected into the formation via an injection well in communication therewith, which displaces petroleum through the formation to another, remotely located well from which it flows to the surface of the earth. While waterflooding recovers an additional quantity of oil economically, water does not displace petroleum efficiently, since water and oil are immiscible and the interfacial tension between water and oil in quite high. Accordingly, as much as 70 percent of the oil originally present in the formation is commonly left even after completion of waterflooding operations.
The use of surfactant waterflooding has been discussed in many prior art references, and numerous field trials have been undertaken employing surfactant-containing fluids. Petroleum sulfonates and other simple organic sulfonate anionic surfactants may be employed in very low salinity formations, but many petroleum formations contain water whose salinity exceeds the level in which petroleum sulfonates may be employed advantageously.
Prior art references suggest the use of surfactants which are both ethoxylated and sulfated or ethoxylated and sulfonated in high salinity environments. Numerous references suggest the injection of a complex mixture of simple anionic surfactants such as petroleum sulfonate and more soluble surfactants such as ethoxylated and sulfated or ethoxylated and sulfonated surfactants. While laboratory tests indicate such complex mixtures effectively displace petroleum in the presence of high salinity water, field application has frequently been disappointing for a number of reasons. One reason for failure of the multi surfactant-containing fluids is related to the different rates of adsorption of the disimilar surfactants from the aqueous fluid as the fluid passes through the subterranean earth formation. Since optimum performance of a multi component surfactant system is achieved only when the various surfactant species are present in a critical ratio, differential adsorption necessarily changes the ratio of the surfactants and therefore renders the fluid less effective or completely ineffective for low surface tension displacement of petroleum. The use of essentially single surfactant fluids, in which the surfactant is an ethoxylated and sulfated or ethoxylated and sulfonated surfactant is also suggested in the prior art, but it has heretofore been difficult to compound a fluid containing substantially only one surfactant which is effective under high salinity conditions for recovering oil from subterranean deposits thereof.
In view of the serious current shortage of petroleum and shortcomings of the prior art techniques, it can be appreciated that there is a significant need for a surfactant waterflooding oil recovery method employing a fluid containing essentially a single surfactant species which efficiently displaces oil from subterranean deposit thereof.